1. Field of the Invention
The invention relates to a backlight unit and a liquid crystal display module (LCDM), and more particularly, to a light concentrating sheet, a backlight unit and a LCDM where a deformation problem in a configuration is prevented.
2. Discussion of the Related Art
Generally, the cathode ray tube (CRT) has been widely used for TV, measuring machines, information terminals and so on. Unfortunately, the CRT has disadvantages on weight, size and so on. To resolve these problems in the CRT; many kinds of flat panel display devices (FPDs), such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescence display (ELD) devices, vacuum fluorescent display (VFD) device, and so on, have been introduced.
Among these devices, LCD devices are widely used for notebook computers, monitors, TV, and so on, because they have excellent capabilities of a small size, a thin profile, light weight and low power consumption. Moreover, they can display high quality and color images and have a large displaying area.
The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules. In other words, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity. Generally, an additional light source is required because the LCD panel is a non-emissive-type display device. Accordingly, a backlight unit is disposed under the LCD panel. The LCD device displays images using light produced by the backlight unit and supplied to the LCD panel.
The backlight unit may be divided into an edge type and a direct type depending on arrangement of light source: In the direct type backlight unit, at least one light source is positioned under an optical sheet. In the edge type backlight unit, one light source is positioned at a side of the backlight unit. Particularly, the light source is positioned at a side of a light guide plate of the backlight unit. In the edge type backlight unit, light emitted from the light source is projected on a light crystal panel using a light guide plate. The light is changed into a plane light source. The backlight unit includes an optical sheet, such as a diffusion sheet and a prism sheet as a light concentrating sheet, to improve optical properties, for example, brightness uniformity.
FIG. 1 is a cross-sectional view of a portion of a related art backlight unit. Referring to FIG. 1, the light concentrating sheet 10 includes a base film 17 and a prism film 15. The prism film 15 has a convex portion 12 and a concave portion 14. The light guide plate 20 is disposed under the light concentrating sheet 10. The prism film 15 is disposed between the base film 17 and the light guide plate 20. The light from the light source 16 at a side of the light guide plate 20 is guided into the light concentrating sheet 10 by the light guide plate 20.
There are some problems. Because the convex portion 12 of the prism film 15 has a peaked shape, there are damages on the prism film 15 and the light guide plate 20 at a contacting portion 18 of the prism film 15 and the light guide plate 20. For example, brightness uniformity may be degraded by particles generated from frictions between the prism film 15 and the light guide plate 20. Moreover, deformation in the prism film 15 and the light guide plate 20 causes a light loss such that light efficiency is reduced. When the base film 17 is disposed between the prism film 15 and the light guide plate 20, there are also problems. For example, since the concave portion 12 of the prism film 15 contacts a diffusion sheet (not shown), there are problems at a contacting portion between the diffusion sheet (not shown) and the prism film 15.